Galaxy Zoo Starburst Talk

Color-Magnitude Chart - an explanation, please!

  • JeanTate by JeanTate

    I understand the terms "Color" and "Absolute Magnitude" - and I'll be writing a post or two on these for fellow Quench zooites who may not - but the default which comes up when I click on this is quite mysterious!

    Which color is being plotted on the y-axis?

    What is "R (mpg)", the x-axis label? To me, "mpg" means "miles per gallon"! 😮

    What does "Distribution Rank" mean?

    What data is populating the chart?

    The colors of the bins/cells presumably reflect the number of subjects objects in each cell, but what's the scale?

    Why does the chart have 'hard' limits on the x- and y-axes scales?

    Putting the cursor over a cell brings up up to three thumbnails (presumably the one and two thumbnail cells mean there are only one/two galaxies in that cell). If there are more than three galaxies in a cell, which three are thumbnailed?

    What are the largish black dots on the y-axis?

    Posted

  • edwardothegreat by edwardothegreat

    Hi Jean, so the Color being plotted is u - g color. The chart is hardcoded to use Abs_R (mag), which I believe is the Absolute Magnitude of the R-band in SDSS. It should also say (mag) instead of (mpg) and I have that typo fixed shortly. I'm not entirely sure Quench subjects have the Abs_R field... so maybe I can rouse a science team member to figure out what would be a good substitute.

    Posted

  • edwardothegreat by edwardothegreat

    Colors are computed from the entire SDSS dataset from the current Galaxy Zoo. The distribution rank is a ranking of the probability a galaxy will be in the bin (so a rank of 1 has the highest probability a galaxy will be there). All the dots are ending up on the Y-axis, because there isn't an Abs_R field in their associated data. otherwise, they would be a plot of where your galaxy is in the color-mag chart.

    Posted

  • JeanTate by JeanTate in response to edwardothegreat's comment.

    Thanks! Some documentation would be nice ... beyond this thread 😉

    If you add data which contains objects with (u-g) outside the fixed range, the y-axis changes accordingly (which is nice). But why the cutoff at (u-g) ~4.0? There most definitely are SDSS galaxies/objects redder than this! Ditto for the Abs_R hard limits.

    Distribution rank is what the colors of the cells is based on, it would seem ...

    Posted